Short answer
Look in the primary filter bowl and at the bottom of the tank. Diesel bug shows as dark brown or black slime, stringy deposits, or a sludge layer at the fuel-water interface, usually with a sour smell. Repeat filter blockages with no other explanation are the strongest practical sign.
You will need: Clear container, torch, sample jar, gloves
Microbial growth lives at the boundary between water and diesel. No water, no bug — which is why every effective treatment starts with removing the water.
What you are looking for
- In the filter bowl: dark slime, stringy or gelatinous material, a sludge layer sitting between water and fuel.
- In a tank sample: a distinct dark interface layer rather than a clean water-to-fuel boundary.
- Smell: sour or rotten, distinct from the ordinary smell of diesel.
- Behaviour: filters that block repeatedly, worse after rough weather, with no other cause.
Take a sample from the bottom of the tank, not the top. A clean top sample proves nothing.
Why treatment alone fails
Biocide kills the organism but does not remove the dead mass, which continues to block filters — and does nothing about the water that let it grow. The order that works is: remove the water, remove the contamination, then treat and control.
See how to get water out of a diesel tank and the full picture in diesel bug and water in fuel.
The Australian context
Tropical humidity and a boat sitting through a wet season are ideal conditions for growth. Boats laid up in the north from November to April and returned to service without checking the fuel are the classic case — see Darwin and Cairns.